The present invention relates to a reflective sensor which projects light emitted from a light-emitting element onto an object to be measured, receives the light reflected thereby at a light-receiving element, and detects displacement of the object on the basis of a change in the amount of the received light.
FIG. 23 is a perspective view showing a reflective encoder in the related art. FIG. 24 shows an XZ section thereof. FIG. 25 shows a YZ section orthogonal to a displacement detection axis corresponding to an X axis. Light rays are radiated from a light-emitting element 1 formed of an LED chip, and some of the light is reflected by a reflective scale 2 and is received by a light-receiving element 3 formed of a photo-IC chip containing a signal processing circuit. The light-emitting element 1 and the light-receiving element 3, each of which is a semiconductor device, are die-bonded to a printed circuit substrate 4 and covered with transparent members constituted by a light-transmissive resin 5 and a transparent glass substrate 6. The light-emitting element 1, the light-receiving element 3, the substrate 4, the resin 5, and the transparent glass substrate 6 constitute a detection head 7.
On the other hand, the reflective scale 2 is formed of a reflective scale base material 8 and a reflective-layer-forming portion 11 including a reflective layer 9 and another reflective layer 10. A reflective encoder of the abovementioned type has been disclosed in Japanese Patent Laid-Open No. 2003-337052.
FIG. 26 is a perspective view showing a detection head part of another reflective encoder in the related art that has been disclosed in Japanese Patent Laid-Open No. 2004-6753. FIG. 27 is a section view thereof. A circuit pattern 12a in a predetermined shape is formed on a substrate 12. A light-emitting element 13 which has a light-emitting region 13a and a light-receiving element 14 which has a light-receiving region 14a and includes a signal processing circuit are die-bonded to the circuit pattern 12a. Each of terminals of the light-emitting element 13 and the light-receiving element 14 is connected through wires 15.
The light-emitting element 13, the light-receiving element 14, and the wires 15 are covered with a surrounding layer 16 made of transparent resin material and a transparent glass substrate 17. As shown in FIG. 27, the surrounding layer 16 needs to have a height equal to or more than those of the light-emitting element 13 and the light-receiving element 14. The thickness of the surrounding layer 16 is determined by taking account of the loop heights of the wires 15 and the margin for bonding of the optical semiconductor components to the substrate 12.
As shown in FIGS. 26 and 27, a light-shield wall 18 is formed approximately at the midpoint between the light-emitting element 13 and the light-receiving element 14. The circuit pattern 12a is placed immediately below the light-shield wall 18. As shown in FIG. 27, the light-shield wall 18 is formed by filling a groove having a width W with a light-blocking resin. A depth d of the groove before it is filled with the light-blocking resin is slightly smaller than the total thickness of the surrounding layer 16 and the transparent glass substrate 17 to protect the circuit pattern 12a on the substrate 12.
The light-shield wall 18 is provided to prevent the light emitted from the light-emitting region 13a of the light-emitting element 13 from being propagated through the surrounding layer 16 and entering the light-receiving region 14a of the light-receiving element 14.
For the light-shield member disposed between the light-emitting element 13 and the light-receiving element 14, various methods have been proposed in generally used devices such as reflective sensors and reflective photointerrupters, and some of the methods have been disclosed in Japanese Patent Laid-Open No. 2000-277796, Japanese Patent No. 3782489, and Japanese Patent 3261280.
The following problems have been found in semiconductor packages for reflective encoders and reflective sensors in the related art. For example, in FIG. 25, light rays included in an angular range θ1 correspond to effective light rays which are introduced from the light-emitting region of the light-emitting element 1 to the light-receiving region of the light-receiving element 3 via the reflective scale 2.
If an angle θ2 is significantly large with respect to a principal ray axis a1 on which the highest intensity of the light-emitting element 1 is provided, the light rays in the angular range θ1 including the inclined light ray at the center thereof account for only a small proportion of all of the light rays radiated by the light-emitting element 1. As a result, most of the light rays are ineffective components, leading to the problem of extremely low use efficiency of light.
FIG. 28 shows an enlarged view of part of FIG. 25. When attention is paid to the light rays between the light-emitting element 1 and the light-receiving element 3 in the package, there is a path of light rays represented by a light ray L which is emitted by the light-emitting element 1 and is reflected by a surface portion 6a of the transparent glass substrate 6. If an angle θ3 is larger than the critical angle (θi), the light ray L is totally reflected and enters the light-receiving element 3. The light ray serves as a large bias light component which is superposed on a sensor signal.
In this case, the substantial S/N ratio of the sensor signal is greatly reduced due to the effective reflected light rays contained in the small proportion and the bias component of light contained in the large proportion. The proportion of the effective light rays can be increased by using a lens. However, the use of the lens causes variations in performance of the sensor resulting from a shift in position between the optical axis of the lens, the light-emitting element 1, and the light-receiving element 3.
Increasing the area of the light-receiving region of the light-receiving element 3 is readily contemplated to increase the proportion of the effective light rays. However, the larger light-receiving region receives a larger amount of the totally reflected light from the surface portion 6a of the transparent glass substrate 6, so that almost no improvement can be achieved in the substantial S/N ratio of the sensor signal. In addition, the larger light-receiving region increases the size of the detection head 7 and causes a disadvantage from an economic viewpoint since the cost is inevitably increased.
Increasing the light amount emitted by the light-emitting element 1 is also a conceivable approach for complementing a small amount of light reaching the light-receiving element 3. However, the power consumption is increased and an excessive amount of current is applied to the light-emitting element 1 to present a problem of reducing the life of the light-emitting element 1. Another conceivable approach is to perform signal amplification in the signal processing circuit, but this increases electrical noise components to affect the accuracy in position detection and thus cannot provide an effective means.
The abovementioned approaches including the use of the lens, the larger light-receiving region, the increased amount of the light emitted by the light-emitting element 1, and the enhanced signal amplification factor in the signal processing circuit on the light-receiving element 3 do not realize significant improvement in performance, or increase variations in performance.
The use of the light-shield means shown in FIGS. 26 and 27 can prevent the bias component of light that is one of the problems in the related art described above and attains a considerable degree of improvement. As shown in a graph of FIG. 29, when the light-shield member is used, an output voltage V of the reflective sensor which provides an analog output varies with a distance G between the reflective sensor and a reflective sample serving as an object to be measured.
Specifically, individual reflective sensors provide different output voltages V at a distance d to a reflective sample shown in FIG. 27 depending on variations in the mounting positions of the light-emitting element 13 and the light-receiving element 14, the position of the light-shield wall 18 placed between them, or variations in the light-receiving sensitivity. In other words, the output voltages show various values as v1, v2 and v3 on the vertical axis of the graph in FIG. 29 depending on the individual differences.
If the reflective sensor is used at a distance equal to or smaller than a distance G1, the sensitivity with the distance between the reflective sensor and the reflective sample is high. Thus, the use of the reflective sensor at such a close distance should be avoided practically, so that an extra space area is required accordingly.
Since the light-shield body serving as the light-shield member is placed at the midpoint between the light-emitting element 13 and the light-receiving element 14, a large interval must necessarily be provided between the light-emitting element 13 and the light-receiving element 14. This increases the area for mounting the elements to prevent a reduction in size of the reflective sensor. In addition, the use of the light-shield member inevitably increases the cost.